Why China has both spy balloons and spy satellites

Driving the news: China's foreign ministry on Monday accused the U.S. of sending high-altitude balloons "illegally" into Chinese airspace more than 10 times since last year.

• U.S. officials deny this. "Any claim that the U.S. government operates surveillance balloons over the PRC (People's Republic of China) is false," National Security Council spokeswoman Adrienne Watson said.

Between the lines: The paths of high-altitude balloons are largely governed by winds. As they climb through the troposphere — where most weather occurs — they're buffeted by west-to-east prevailing winds.

• Those winds make it relatively easy for China to launch balloons that would fly above the U.S., but much harder for the U.S. to fly balloons over China.
• "Where are you launching them from? That gets left out of this a lot," James Lewis of the Center for Strategic and International Studies, tells Axios.
• "Taiwan? Winds don't blow that way. Korea? Winds don't blow that way. Japan? You could do it if you didn't mind a bunch of them going off course. You could do it, but I think the Chinese are either confused or making this up."

How it works: U.S. spy satellites were developed after high-altitude balloons and aircraft began to be targeted in enemy airspace.

• Satellites can provide exquisite imagery and collect signals — from communications systems and other technology — but they are relatively easy to track, even by amateurs on the ground.
• High-altitude balloons, however, can gather high-resolution images and signals, and fly in a part of airspace that make them relatively difficult to track. Balloons are also able to stay over one area for a longer time than a satellite.

Yes, but: The norms that govern spying in space and airspace, where high-altitude balloons fly, are different.

• The difference between orbit and airspace was established after the Soviet Union launched Sputnik in 1957.
• Sputnik "goes up and goes around and nobody objects — including the United States — to that overflight," the Secure World Foundation's Brian Weeden tells Axios. "That establishes precedent that satellites can go round and round and they have this freedom of overflight — that space is legally different than airspace."
• That means shooting down unknown foreign objects in a nation's airspace is tolerated today, whereas blowing up a spy satellite would likely be considered a huge escalation.

The U.S. and China have different strategies for their spy satellite networks.

• The U.S. has historically used a relatively small number of expensive, technically advanced satellites that take incredibly detailed images of Earth.
• China, meanwhile, has a more distributed network of spy satellites that aren't quite so exquisite in their imagery but it's a "good enough approach," Weeden says.
• The U.S. is interested in moving toward a more distributed spy satellite network, making it more resilient to attack and making the military less reliant on a small number of expensive satellites that could be appealing targets for jamming, dazzling and other interference.

The intrigue: The high-altitude balloon dispute may also call into question China's spy satellite capabilities, Lewis says.

• If the balloon shot down on Feb. 4 was collecting signals intelligence, it may mean China's spy satellites are limited in what information they can collect, he says.

What to watch: A big question looms over 21st century space-based defense about defining where airspace ends and space begins.

• Currently, there isn't a legal, accepted definition of where space begins and airspace ends internationally.
• Some nations "want to be able to exploit this gray zone between air and space," Weeden said, adding that's because these countries are conducting activities in that grey zone they don't want them to fall into either airspace or space.